Eruption controls

The shapes and lithological characteristics of glaciovolcanic landforms are influenced by a number of factors:

Composition controls many lava properties, including the liquidus temperature, glass transition temperature (below which crystallization cannot occur), viscosity, volatile content, and heat capacity. These properties in turn influence eruption style and effusion rates. Compositional differences, thus, manifest themselves as differences in landform shapes and internal characteristics. For example, in British Columbia's Garibaldi Volcanic Belt, basaltic lavas typically form conventional-style tuyas, consisting of pillows overlain by hyaloclastites overlain by flat-lying subaerial lava flows. Intermediate composition glaciovolcanic edifices, however, form flat-topped, steep-sided piles of flows with little or no fragmental material; we call these flow-dominated tuyas.

Eruption parameters such as temperature, effusion rate, and length of the eruption also influence the final shapes, sizes, and internal characteristics of glaciovolcanic edifices (composition, discussed above, influences many of these eruption parameters). Effusion rate influences transfer of heat from the cooling lava to its surroundings, and thus controls the amount of meltwater produced during the eruption. Longevity of the eruption influences whether a volcano is able to melt a hole through the overlying ice sheet; small-volume, short-lived eruptions under thick ice are less likely to melt completely through the glacier to erupt subaerially.

Vent type is important to the shape of the final edifice. Fissure eruptions lead to the formation of hyaloclastite ridges, whereas central vent eruptions result in tuyas, subglacial mounds, and subglacial domes.

Ice characteristics influence the drainage or accumulation of meltwater during subglacial eruptions. Where ice is thin, subglacial volcanic edifices are likely to breach the ice surface and be capped by subaerial flows. Additionally, glaciers that are frozen to their beds, or are thick and impermeable, favor the accumulation of meltwater, which results in lithological features formed by subaqueous eruption (such as pillows and hyaloclastites). Glaciers that can be floated above their beds, or are thin and permeable, favor the escape of meltwater. In some cases, this can lead to the formation of landforms which are shaped more by overriding ice than by direct lava-water interaction. For example, in British Columbia's Garibaldi Volcanic Belt, intermediate composition subglacial domes show little evidence for direct lava-water interaction, and their shapes may reflect the shapes of the ice cavities into which they were erupted.

The nature of bedrock or surrounding topography influences the development of glaciovolcanic edifices in several ways. Firstly, steep topography coupled with high altitude vents is likely to promote meltwater drainage. Irregularities in bedrock topography may also allow water to accumulate at or near subglacial vents, particularly in areas which are locally relatively flat. Secondly, extreme topography during continental-scale glaciations may result in high altitude vents, which are thus overlain by thinner ice than that which overlies valley bottoms. If high-altitude vents are ice-free, lavas may flow downhill to pool against valley-filling ice, resulting in numerous impoundment features. Such features are very common in British Columbia's Garibaldi Volcanic Belt, which has steep and irregular topography. Finally, steep topography enhances erosion rates, which can result in removal of unconsolidated fragmental material, as well as landsliding of steep, unstable lava flow boundaries.

Factors controlling the shape and internal characteristics of glaciovolcanic edifices are shown schematically below: